Return error if implementationOnlyImports is enabled with public visibility

Author: gjcairo

Sources/protoc-gen-swift/FileGenerator.swift

@@ -90,6 +90,15 @@ class FileGenerator {
 
         p.print("\(comments)import Foundation")
 
+        if self.generatorOptions.implementationOnlyImports,
+           self.generatorOptions.visibility == .public {
+            errorString = """
+                Cannot use @_implementationOnly imports when the proto visibility is public.
+                Either change the visibility to internal, or disable @_implementationOnly imports.
+            """
+            return
+        }
+
         // Import all other imports as @_implementationOnly if the visiblity is
         // internal and the option is set, to avoid exposing internal types to users.
         let visibilityAnnotation: String = {